1. Field of the Invention
Embodiments of the present invention relate generally to micro-electro-mechanical and nano-electro-mechanical systems and more specifically to an anti-stiction and lubrication for such systems.
2. Description of the Related Art
As is well-known, atomic level and microscopic level forces between device components become far more critical as devices become smaller. Micromechanical devices, such as Micro-electro-mechanical systems (MEMS) and nano-electro-mechanical systems (NEMS), are an area where problems related to these types of forces are quite prevalent. In particular, “stiction” forces created between moving parts that come into contact with one another, either intentionally or accidentally, during operation are a common problem with micromechanical devices. Stiction-type failures occur when the interfacial attraction forces created between moving parts that come into contact with one another exceed restoring forces. As a result, the surfaces of these parts either permanently or temporarily adhere to each other, causing device failure or malfunction. Stiction forces are complex surface phenomena that generally include capillary forces, Van der Waal's forces and electrostatic attraction forces. As used herein, the term “contact” refers generally to any interaction between two surfaces and is not limited to the actual physical touching of the surfaces. Some examples of typical micromechanical devices are RF switches, optical modulators, microgears, accelerometers, worm gears, transducers, fluid nozzles, gyroscopes, and other similar devices or actuators.
The stiction issue is especially problematic in devices such as the RF switch, optical modulator, microgears, and other actuators. Various elements in these devices often interact with each other during operation at frequencies between a few hertz (Hz) and about a few gigahertz (GHz). Various analyses have shown that, without adding some form of lubrication to these types of devices to reduce stiction and wear between component surfaces, product lifetimes may range from only a few contacts to a few thousand contacts, which is generally well below a commercially viable lifetime. Consequently, one of the biggest challenges facing the MEMS and NEMS industries is the long-term reliability of contacting microstructures in the face of stiction.
Several techniques to address the stiction between two contacting surfaces have been discussed in the various publications. These techniques include texturing the surfaces (e.g., micro patterning or laser patterning) to reduce the overall adhesion force by reducing the effective contact area, and selecting specific materials from which the contacting surfaces are made to lower the surface energy, reduce charging, or contact potential difference between components.
Moreover, some prior references have suggested the insertion of a “lubricant” into the region around the interacting devices to reduce the chance of stiction-related failures. Such a lubricant often times is in a solid or liquid state, depending on the properties of the material, and the temperature and pressure or environment in which the lubricant is placed. In general, the terms a “solid” lubricant or a “liquid” lubricant is a lubricant that is in a solid or liquid state under ambient conditions, which is typically defined as room temperate and atmospheric pressure. Some prior art references describe a lubricant as being in a “vapor” state. These references use of the term vapor phase lubricant to generally describe a mixture of components that contain a carrier gas (e.g., nitrogen) and a vaporized second component that is a solid or liquid at temperatures and pressures near ambient conditions (e.g., STP). In most conventional applications the solid or liquid lubricant will remain in a solid or liquid state at temperatures much higher than room temperature and pressures much lower than atmospheric pressure conditions.
Another common approach to combat stiction between interacting components is to coat the various interacting components with a low-surface energy organic passivation layer, such as the self-assembled monolayer (SAM). The low-surface energy organic passivation layer coating results in a hydrophobic surface that is used to reduce or eliminate capillary forces, molecular bonding forces, and reduce electro-static attraction forces in some cases. The material(s) used to form a SAM layers are typically liquids under ambient conditions. Self-assembled-monolayer coatings are commonly applied to MEMS type devices by immersion of the device in a liquid containing the components used to form the SAM molecules. In some cases low-surface energy organic passivation layer, such as a SAM coating, can be formed by exposing the surface of the device to a vapor containing a carrier gas that has SAM layer forming components entrained in it typically by bubbling the carrier gas through a vessel containing heated SAM layer forming components.
Another common approach to combat stiction between interacting components is to use a nebulization process that uses a liquid lubrication system that creates a lubricant “fog,” or lubricant “mist,” that lubricates the surfaces of the MEMS device by exposing the interacting surfaces to tiny droplets of the liquid lubricant that is suspended in a carrier gas. One such process is described in column 3, line 28 of U.S. Pat. No. 6,921,680, where it notes that “it is critical that the nebulizer system be maintained in a homogenous cloud of the lubricant around the device specimen.” These types of systems require additional steps to keep the concentration of the liquid droplets within the lubricant “fog” homogeneous which can be complex and costly. The use of the lubricant “fog” will also require additional processing time to lubricate the devices to ensure that the “mist” reaches all parts of a device to form a suitable lubrication layer.
Examples of typical lubricants that are solid or liquid at ambient conditions and temperatures well above ambient temperature can be found in reference such as U.S. Pat. No. 6,930,367. Such prior art lubricants include dichlorodimethylsilane (“DDMS”), octadecyltrichlorosilane (“OTS”), perfluoroctyltrichlorsilane (“PFOTCS”), perfluorodecanoic acid (“PFDA”), perfluorodecyl-trichlorosilane (“FDTS”), perfluoro polyether (“PFPE”) and/or fluoroalkylsilane (“FOTS”) that are deposited on various interacting components by use of a vapor deposition process, such as atmospheric chemical vapor deposition (APCVD), low pressure chemical vapor deposition (LPCVD), plasma enhanced chemical vapor deposition (PECVD), or other similar deposition processes.
The technique of forming the low-surface energy organic passivation layer on the surface of MEMS/NEMS components is commonly referred to in the art as “vapor lubricant” coating. One serious draw back to using low-surface energy organic passivation layer, such as SAM coatings, is that they typically are only one monolayer thick, although coatings that are a few monolayers have also been reported. Generally, these types of coatings have a very limited usable lifetime, since they are easily damaged or displaced due to impact or wear created by the interaction of the various moving components. This is inevitably happens in MEMS devices with contacting surfaces, such as light modulator and RF switches, that are subject to frequent contact in use and a large number of contacts during the product lifetime. Without some way to reliably restore or repair the damaged coatings, stiction inevitably returns, and device failure results.
One approach for repairing the damaged contacting area, coating being displaced or broken down (FIG. 1A) is to provide a getter 110 within the package 100 (that includes a base 102, a lid 104, and a seal 106) in which an array of MEMS devices 108 resides. FIG. 1B illustrates one conventional package 120 that contains a MEMS device 108 and a getter 110 which are positioned within the head space 124 of the package 120. The package 120 also contains a package substrate 128, window 126 and spacer ring 125. These two configurations are further described in U.S. Pat. No. 6,843,936 and U.S. Pat. No. 6,979,893, respectively. As previously indicated, these conventional devices employ some type of reversibly absorbing getter to store the lubricant molecules in the zeolite crystals or internal volume of a micro-tube. In these types of designs, a supply of lubricant is maintained in the getter 110 and an amount of lubricant needed to lubricate the MEMS device 108 is discharged during normal operation. However, adding the reversibly absorbing getter, or reservoirs, to retain the liquid lubricants increases package size and packaging complexity and adds steps to the fabrication process, thereby increasing piece-part cost as well as the overall manufacturing cost of MEMS or NEMS devices. Thus, forming a device that uses these techniques generally requires a number of labor-intensive and costly processing steps, such as mixing the getter material, applying the getter material to the device containing package, curing the getter material, conditioning or activating the getter material, and then sealing the MEMS device and the getter within the sealed package.
As the foregoing illustrates, what is needed in the art a more reliable and cost-effective approach to providing anti-stiction lubrication to MEMS and NEMS, particularly for the in-use replenishment of the lubricants, in addition to the initial coating of the MEMS/NEMS device with a lubricant.